C isn't going away. POSIX isn't going away. Multithreaded code written in C for POSIX isn't going away. So pthreads isn't going away.
Many implementations of std::thread will use pthreads under the hood.
"The Pthreads API is defined in the ANSI/IEEE POSIX 1003.1 - 1995 standard." -- POSIX Threads Programming https://computing.llnl.gov/tutorials/pthreads/